Photomasks are used in the art of photolithography for producing printed circuits and other precision photo-fabricated parts. In a photolithographic process a substrate is covered with a layer of photoresist in which a pattern is photographically developed by superimposing over the photoresist a photomask having patterned transparent and opaque areas and then passing actinic radiation, usually ultraviolet light, through the transparent areas of the photomasks. A pattern is then developed in the photoresist as a relief image by means of differential solubilities of the exposed and unexposed portions. Etching or other treatments may then be carried out on the underlying exposed portions of the substrate. A general discussion of the state of the art of photo-fabrication and the role played by photomasks may be found in Scientific American, September 1977, pages 111-128.
Photomasks have typically consisted of films of photographic emulsion, iron oxide, or chromium on glass plates. However, since a photomask is essentially a production tool which must be used repetitively it has been desired to increase the durability of photomasks by creating the desired opaque and transparent areas as stained images within the glass plates rather than coatings on the surfaces of glass plates. Several techniques for producing stained images within glass plates are known, such as those disclosed in U.S. Pat. Nos. 3,573,948 (Tarnopol); 3,732,792 (Tarnopol et al.); 3,561,963 (Kiba); 2,927,042 (Hall et al.); 3,620,795 (Kiba); 4,144,066 (Ernsberger); and 4,155,735 (Ernsberger) and in pending U.S. patent applications Ser. No. 60,422 filed July 25, 1979 and Ser. No. 80,875 filed on Oct. 1, 1979, both by Fred M. Ernsberger.
The present invention relates to an improvement in any of these techniques or any other technique for producing stained patterns within glass plates or sheets wherein one of the essential steps entails heating the glass to a relatively high temperature. Developing a stain within the glass generally requires subjecting the glass to a temperature above 300.degree. C. and sometimes as high as 500.degree. C. or above. It has been found that at these elevated temperatures the glass substrate undergoes permanent dimensional changes which cause the dimensions of the stained pattern to deviate from those intended. Because the patterns carried by photomasks must be precisely predetermined even a small degree of such dimensional inaccuracy may be unacceptable. With standard soda-lime-silica flat glass employing a heat treatment which may range as high as about 525.degree. C., shrinkage of the glass is encountered. The rate of shrinkage is slower at lower temperatures and may be insignificant at temperatures around 300.degree. C. for reasonably short heat treatment times. However, it is generally preferred to maximize the temperature so as to shorten the length of a heat treatment step. Also, the production of more durable, deeply penetrated stains is sometimes associated with heat treatment at higher temperatures. Thus, avoiding dimensional instabilities appeared to be at odds with other desirable factors in the production of stained glass photomasks.
It has been possible to photographically compensate for the dimensional instabilities of the glass when photographically transferring the image to the glass, but such a procedure is not very satisfactory in most cases due to loss of resolution in the resulting image. Compensation for dimensional changes could also be made in the original design layout, but it would be difficult to predict the precise amount of change which would be encountered and such a procedure would be cumbersome to carry out.